The present invention relates to a polishing composition for a substrate for a magnetic disk, to be used for the production of a magnetic disk which is used as a memory device for e.g. a computer, and a polishing method employing it.
There have been continuing efforts for miniaturization, larger capacity and lower price for magnetic disks such as memory hard disks to be used for computers or electrical household appliances. At present, a substrate for a magnetic disk (hereinafter referred to simply as a substrate) to be used for the preparation of such a magnetic disk, is prepared by forming a plating film such as an electroless Nixe2x80x94P plating film on a substrate (a blank material) made of a metal material such as aluminum.
In such a case, the plating film will be formed along a waviness formed at the time of forming the blank material or along irregularities on the surface of the blank material, and such a waviness or irregularities may sometimes remain on the substrate surface. Accordingly, for the purpose of removing such waviness and irregularities to make the substrate surface flat and smooth, polishing of the substrate is carried out by means of a polishing composition for a substrate for a magnetic disk (which may hereinafter sometimes be referred to simply as a polishing composition).
Along with the increase in the capacity of magnetic disks in recent years, the surface quality required for the substrate surface after the polishing has become increasingly strict, and the surface roughness is presently required to be Ra=at most 10 xc3x85 as surface roughness measured by AFM (manufactured by Digital Instruments Company (U.S.A.) as a feeler type surface roughness meter, although it may vary depending upon the grades of the magnetic disks.
Accordingly, the precision required for the substrate surface after the polishing has become extremely strict, and a polishing composition is desired whereby a more flat and smooth polished surface can be obtained.
For the above-mentioned purpose, polishing has heretofore been carried out by means of a polishing composition comprising aluminum oxide or various other abrasives, water and various polishing accelerators. For example, JP-A-61-278587 and JP-A-62-25187 disclose a polishing composition for a memory hard disk, which is prepared by adding aluminum nitrate, nickel nitrate or nickel sulfate as a polishing accelerator to water and aluminum hydroxide, followed by mixing to obtain a slurry.
Further, JP-A-2-84485 discloses an acidic polishing composition for an aluminum magnetic disk, which comprises water and an alumina abrasive powder as well as gluconic acid or lactic acid as a polishing accelerator and colloidal alumina as a surface modifier, JP-A-7-133477 discloses an aqueous polishing composition comprising an alumina abrasive, colloidal alumina and an alkali nitrite, and JP-A-9-316430 discloses a polishing composition for a magnetic disk substrate, which comprises water, xcex1-alumina and a polishing accelerator, wherein the polishing accelerator is aluminum oxalate.
Further, those having constructions as disclosed in JP-A-7-216345 and JP-A-2000-1665 are also known. The former polishing composition comprises water, an alumina abrasive and a polishing accelerator composed of a molybdate and an organic acid, while the latter polishing composition comprises water and at least one abrasive selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide, silicon nitride and manganese dioxide and further contains succinic acid or its salt dissolved in this composition.
Each of these polishing compositions of conventional constructions has a high stock removal rate and is designed to suppress formation of surface defects such as microprotrusions and fine pits on the substrate surface by carrying out polishing of the substrate by means of each polishing composition.
On the other hand, in order to meet the requirement to increase the capacity of a memory hard disk, it is desired not only to make the polished surface more flat and smooth but also to effectively utilize the peripheral portion (the edge portion) of a memory hard disk to enlarge the memory area thereby to increase the memory capacity.
However, with the polishing compositions of conventional constructions, if polishing of a substrate is carried out by means of such a polishing composition, edge sagging may result at the edge portion of the substrate, whereby there has been a problem that the edge portion of the memory hard disk can not effectively be utilized. With a substrate having sagging formed at the edge portion, it will be difficult for a magnetic head designed to read out and write memory information to maintain the distance (the flying height) from the substrate which is rotated at a high speed, and it becomes impossible to record data at such an edge portion, thus leading to a problem that the memory region for data will be reduced.
The present invention has been made in view of such a problem in the prior art, and it is an object of the present invention to provide a polishing composition for a substrate for a magnetic disk, whereby formation of edge sagging at the edge portion of a substrate during polishing, is suppressed thereby to secure a wider memory region by effectively utilizing the edge portion of the memory hard disk and whereby a substrate can be polished at a high stock removal rate, and formation of surface defects in the substrate for a magnetic disk after the polishing, can be suppressed, and a polishing method employing such a composition.
For the purpose of accomplishing the above object, the present invention provides 1) a polishing composition for a substrate for a magnetic disk, which comprises (a) a polishing accelerator composed of at least one compound selected from the group consisting of malic acid, glycolic acid, succinic acid, citric acid, maleic acid, itaconic acid, malonic acid, iminodiacetic acid, gluconic acid, lactic acid, mandelic acid, crotonic acid, nicotinic acid, aluminum nitrate, aluminum sulfate and iron(III) nitrate, (b) an edge sagging preventive agent composed of at least one compound selected from the group consisting of a polyvinylpyrrolidone, a polyoxyethylene sorbitan fatty acid ester and a polyoxyethylene sorbit fatty acid ester, (c) at least one abrasive selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide and silicon carbide, and (d) water.
Further, the present invention provides 2) the polishing composition for a substrate for a magnetic disk according to the above 1), wherein the polyoxyethylene sorbitan fatty acid ester and the polyoxyethylene sorbit fatty acid ester in (b) are at least one compound selected from the group consisting of polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate, polyoxyethylene sorbitan trioleate, polyoxyethylene sorbitan monocaprylate and polyoxyethylene sorbit tetraoleate.
The present invention also provides 3) the polishing composition for a substrate for a magnetic disk according to the above 1), wherein the polyoxyethylene sorbitan fatty acid ester and the polyoxyethylene sorbit fatty acid ester in (b) are such that the proportion of addition of ethylene oxide is at most 30 per molecule of sorbitan or sorbit; 4) wherein the polyvinylpyrrolidone in (b) has an average molecular weight of from 2,500 to 2,900,000; or 5) wherein the content of the component (b) is from 0.001 to 2 wt % based on the total weight of the polishing composition.
Further, the present invention provides 6) the polishing composition for a substrate for a magnetic disk according to the above 1), wherein the content of the component (a) is from 0.01 to 25 wt % based on the total weight of the polishing composition; and 7) a polishing method for a substrate for a magnetic disk, which comprises polishing a substrate for a magnetic disk by means of the polishing composition as defined in any one of the above 1) to 6).